1. Field of the Invention
This invention relates to brushless motors, and in particular to linear brushless motors.
2. Description of the Related Art
The history of linear motor technology dates far into the past. To simplify, this will be divided into the electromagnetic design and the commutation. With regard to the electromagnetics, motor technology proceeded primarily in rotary form. The linear motor form is also old, this can further broken into actuators and linear motors. Actuators may be defined as short linear or rotary devices and have unique structures not feasible for extended motion. Linear motors are defined here as electromagnetic devices which need or use commutation to extend their range of useful stroke.
The electromagnetic design of linear motors follows the well-known laws of classical electromagnetic theory. Early devices are like the common voice coil which is of the ironless armature type. They are efficient, have control linearity, but require considerable magnetic material, are not low cost nor lightweight. They are suitable for certain applications and are included in the scope of this application. The ironless armature motor in rotary form has a heritage in fast response servo systems and were utilized at Goddard Space Flight Center (GSFC) in the early 1970s in flywheels for low loss rotation. They are readily implemented in linear form and minimize the magnetic design but few applications reach linear velocities which warrant their large magnet requirement to drive flux across an airgap wide enough to accommodate the armature windings.
The more general form of d.c. motors its armature windings placed in slots of laminated steel which provides a low reluctance path for magnetic flux or sometimes wound directly on their (not slotted) surface, These may be planer surfaces mechanically supported to maintain a uniform gap while allowing axial motion, or other geometries including cylindrical concentric shapes to minimize “end turns” and stray magnetic field losses. In virtually every case, two elements are involved, armature assembly and the field assembly. This was the case until the d.c. motor with stationary armature and field was developed at the Goddard Space Flight Center as described in U.S. Pat. No. 3,569,804. This machine has its field assembly divided into two parts (1) the source of field flux which can be physically attached to the armature assembly and (2) a passive salient pole structure.
Progress thru the past 30 years has been primarily driven by improved materials especially SmCo (samarium-cobalt), and NdFeB (neodymium) magnets which are vastly more powerful. As in the past, linear motor design followed the much more common rotary machines.
The commutation of linear motors trailed far behind that of rotary devices due to the mechanical difficulties of implementation. The advent of electronic commutation in 1962; See NAS 5-2108. Development of a Brushless D.C. Motor, changed commutation from a mechanical problem of sliding contacting surfaces to one of electronics which has seen tremendous strides not only in size and capability but more recently in power handling capability. (For example, a 160 Amp IGBT that was recently marketed by IR Corp.) The same company also sells small integrated circuit three phase, high voltage drivers for motor drives. Thus the problem for the linear motor designer today is the selection and placement of suitable position sensors, the proper choice of IC (integrated circuit) switches, and the electromagnetic and electromechanical design of the linear motor itself.